Americans in the Midwest and south United States are bracing for what meteorologists call a “polar vortex,” frigid, dense air that is threatening to shatter temperature records and send wind chills to dangerous levels. The forecast calls for temperatures 15 degrees below zero in Chicago, Indianapolis and Minneapolis, 21-below in Madison, Wisconsin and more than 30-below in Fargo, N.D. The wind chill – what the air feels like once wind is factored in – are estimated at 40 to 50 degrees below zero.

Temperatures in about half the country could be below-zero by Wednesday, CNN reports.

Officials in Chicago, St. Louis and Milwaukee have closed schools, and school systems have announced delays throughout the south. Flight delays are persisting across the country after thousands of flights were already cancelled and delayed because of the earlier blizzard, creating a travel backlog at airports. Overnight temperatures in Atlanta and Birmingham were set to be in the single digits, and Jacksonville, Fla., will see temperatures dip to about 20 degrees, the Weather Channel reports.

Officials caution against long periods of time outside and are reminding all to dress adequately and in several layers, as frostbite and hypothermia will be a real danger at such low temperatures.